The present invention relates to high speed solenoid actuators, referred to herein as "solenoids", and more particularly to methods and apparatus for controlling electronic systems which drive such high speed solenoids.
In order to design high speed solenoids for solenoid control systems, it is necessary to provide electronic control systems capable of driving such solenoids at very high speeds with high efficiency and with nominal average current requirements, particularly as applied to pulse width modulated (PWM) solenoid valve applications. Present PWM solenoid valves are limited to a maximum frequency of operation on the order of 100 to 150 Hz, with open to close travel times on the order of 1.5 milliseconds. The present invention makes possible the design and operation of PWM solenoid valves at frequencies of 400 Hz and above with open to close travel times of the order of 400 microseconds.
In order to design a high speed driver it is first necessary to understand some fundamental concepts of solenoid design. The force developed by a solenoid, for a given gap and neglecting saturation, is proportional to the square of the product of the turns and the current passing through the solenoid coil. To increase the rise time of the force it is necessary to increase the rise time of the current. The rise time of the solenoid current, neglecting eddy currents, circuit resistance, and back electromotive force (emf), is inversely proportional to the applied potential and directly proportional to the solenoid inductance. If the applied potential is fixed, then the inductance must be decreased by decreasing the number of turns on the solenoid coil. However, to maintain the same force level, the current must be increased.
It may be shown that the rise time of the solenoid is proportional to the number of turns for the conditions assumed above. That is, if the turns are cut in half, the current to obtain the same force will be doubled and the inductance will be one fourth of the original value. If the inductance is reduced four times, then a time T to reach the original value of current is reduced to T/4. However, the current must be doubled to produce the original force so an additional increment of time T/4 is required. Therefore the new rise time is one half of the original.
If the solenoid current rises at a sufficiently high rate, it can attain its maximum prescribed level before significant solenoid armature motion occurs. Back emf which is generated due to solenoid armature movement then has almost no effect on solenoid current during its rise period. Consequently, more solenoid force can be supplied during the period of solenoid current rise if the solenoid has a current rise time significantly less than the solenoid armature response time, so that back emf due to solenoid armature motion becomes insignificant.
To attain a very high speed solenoid the inductance must be made low enough so that the desired rise time may be achieved. This implies using very few turns, and hence the coil resistance is very small. Therefore, the steady state current must be limited by external circuit means.
Another requirement to achieve high speed operation is the minimization of eddy currents in the metal parts of the solenoid. This may be done by using appropriate magnetic laminations to restrict the flow of the eddy currents.
To effectively drive the high speed solenoid the electronic driver must use a fast acting solid state switch such as a MOSFET. This switch must be capable of turning on rapidly to apply the input potential, less any switch drop, directly across the solenoid to achieve the rapid rise of current. The driver must then be capable of efficiently limiting the solenoid current as described above. As explained below, it may be necessary to provide more than one level of current as well as a means of achieving rapid transition between these levels.